My products are available in different weights, which have been set up with checkbox options.

I want the customer to be forced to click one of these checkboxes.

This page: http://www.zen-cart.com/content.php?...e-causing-this

says
To force selection of an attribute, you need to:
a. add an attribute that is Display-Only (ie: a color swatch that says "Please Select a Color").
b. Make it default.
This forces the customer to choose something "other than" the default (since the "default" is set to "display only").
This does not work on my 1.5.4 setup using the default template. The customer can click "Add to Cart" and the product is added to the cart without any attributes.

Using a text field option, I get the required effect, but then there is an unwanted text box on the page.